1. Field of the Invention
This application relates to a valve for a high pressure system and, more specifically, to a valve for use with a high pressure cylinder that incorporates a devices structured to reduce the rate of pressurization of a downstream device.
2. Background Information
Valves coupled to a high pressure gas cylinders are well known in the art. Such valves typically open in a single stage. That is, when a user operates an opening mechanism, typically a handle or knob, a valve assembly blocking the flowpath between the valve inlet and outlet moves from a closed position to an open position. When the flowpath is initially opened there is adiabatic compression of the gas through the valve and any downstream devices. While this adiabatic compression of the gas does not effect the valve, it has been determined that certain devices coupled to the downstream side of the valve may be adversely effected. For example, a downstream device may ignite and burn.
One area of special concern is cylinders which hold oxidizers such as, but not limited too, high pressure oxygen cylinders used in medical applications or cylinders containing NO2, hereinafter “oxygen cylinders.” When the oxygen cylinder is used in a medical application, the downstream device coupled to the valve is often a regulator. A regulator steps down the pressure of the oxygen so that the oxygen may be supplied at a breathable pressure. However, with respect to the possibility of a fire, when a critical amount of oxygen, fuel, and heat are combined, a fire will result. The exact critical amount of each of these essential components of a fire differ based upon the characteristics of the regulator, or any other downstream device. That is, by way of example only, a regulator may have a set of characteristics, such as, but not limited to, being made from a specific material, having a thin wall, a specific flowpath and being exposed to a supersonic flow of pure oxygen. Given these characteristics, a properly designed regulator should not ignite as the critical amounts of oxygen, fuel, and heat required for a fire should not be present. However, if any one of the characteristics were changed, again by way of example only, such as by changing the material from which the regulator is constructed, the combination of oxygen, fuel, and heat may be enough to ignite the new material.
In the past, the material of choice for constructing regulator bodies was brass. Brass regulators, in an almost pure oxygen environment, resist burning at a pressure as high as 10,000 psig. Accordingly, when such brass regulators were used with prior art valves there were few, if any, fires related to adiabatic compression. In an effort to reduce the weight of the regulators, some regulator bodies are now made from aluminum and other non-metallic components. Aluminum can burn, in an almost pure oxygen environment, at a pressure as low as 25 psig. On rare occasions, the newer aluminum regulators have caught on fire immediately after the valve has been opened. Such fires are extremely dangerous, first because metal fires, and especially aluminum fires, are extremely hot and second, because a medical oxygen cylinder is typically close to a patient or a caregiver. While such regulator fires have been related to the pressure surge that occurs when the valve is opened, the ignition mechanism is unknown. Certainly, the oxygen enriched environment enhances the chance of a fire once ignition occurs.
While there may be more than one cause of ignition, or a combination of causes, one theory is that the adiabatic compression of the gas within the attached aluminum regulator following the opening of the valve causes a rise in the temperature of the gas within the regulator. This rise in temperature, along with the geometry of the regulator, the shape of the flowpath, and other characteristics of the regulator, may be enough to allow the aluminum regulator to ignite. The chance of igniting the regulator or, if the valve is used for a different purpose, any other downstream device, is increased due to the enriched oxygen environment. This type of ignition is hereinafter identified as “heat of compression ignition.”
Accordingly, one theory as to how to prevent heat of compression ignition is to suppress the adiabatic compression of the gas. Devices structured to reduce the adiabatic compression of the gas are disclosed in U.S. Pat. Nos. 2,367,662, 3,841,353, 4,172,468 and application 2002/0056479 A1. These devices are structured to limit the initial flow of fluid through the valve. Each of these devices, however, have deficiencies. The U.S. Pat. Nos. 2,367,662 and 4,172,468 devices, for example, are separate units that are disposed between the valve and downstream equipment. As such, there is no guarantee that the device will always be used. The U.S. Pat. Nos. 3,841,353 and 2002/0056479 A1 devices are incorporated into the valve, but rely on springs to position the valve element. Such springs may wear out over time or be lost during maintenance of the valve. The device disclosed in application 2002/0056479 A1 also has the disadvantage of having the primary valve close prior to the secondary valve. Thus, it is possible that a user attempting to close the valve, and who may be guided by the audible sound of gas flowing through the primary valve, will only close the primary valve while allowing the secondary valve to remain open. Additionally, certain valves include markings on the outer portion of the housing indicating the position of the valve assembly. In valves that rely on springs for positioning the valve assembly, degradation of the spring may cause the valve assembly to shift relative to the external markings thereby making the markings irrelevant or misleading.
Another suspected cause of ignition of a regulator, or any other downstream device attached to an oxygen cylinder, is particulate matter disposed within the oxygen cylinder. Particulate matter may pass through, or into, the downstream device at high speed upon opening the valve. That is, the opening of a valve may cause a momentary supersonic fluid flow through, or into, the downstream device. A particle within the oxygen cylinder may be picked up by this high speed flow and pass into the downstream device. Again, the chance of ignition depends on the characteristics of the regulator or downstream device. For example only, a downstream device having a non-aerodynamic flowpath with thin protrusions extending into the flowpath, and which are made from a material having a low ignition temperature, is more likely to suffer an ignition due to particle impingement than a device with an aerodynamic flowpath. That is, with a non-aerodynamic flowpath, the particle is more likely to impinge upon the material in such a way as to transfer energy, and therefore heat, to the device housing. This heat, along with the enriched oxygen environment and other characteristics of the device, may cause the particles, and subsequently the device, to catch on fire. This type of ignition is hereinafter identified as “particle impact ignition.”
Additionally, with respect to particle impact ignition, the chance of such an particle impact ignition is increased if the number of particles passing through the valve is increased. A gas cylinder is typically funnel shaped at the end where the valve is attached. This shape is conducive to creating a gas stream that, depending upon the orientation of the cylinder, draws particles directly into the valve inlet. That is, it is less likely that a taller cylinder in vertical orientation will pass particles through a valve than a shorter cylinder in a horizontal orientation. Still, any cylinder that is contaminated with particulate matter may pass those particles into or through the valve.